This application relates to optical amplifiers, and more specifically, to fiber optical amplifiers for wavelength-division multiplexed (WDM) applications.
Optical fibers may be doped with active ions such as rare earth ions to operate as optical gain media. Under proper optical pumping conditions, such a doped fiber may be optically pumped at a pump wavelength within the absorption spectral band of the doped fiber and emit light at an emission wavelength that is longer than the pump wavelength and within the emission spectral band of the doped fiber. When an input optical signal at the emission wavelength is injected into the optically-pumped doped fiber, the input optical signal is amplified by the active ions which convert the pump energy into the optical energy at the emission wavelength.
Doped fibers may be designed to produce optical gain over a limited spectral range. In WDM applications, different optical channels at different WDM wavelengths may be amplified by using the same doped fiber amplifier. One commonly used fiber amplifier for the 1.5-micron band is the Erbium-doped fiber amplifier (EDFA). The typical bandwidth of some conventional EDFAs is about 30 nm and hence is much less than the low-loss bandwidth of commercial fibers. In order to fully use the available bandwidth of the fibers and to meet the demands for broad bandwidth in fiber communications, it is desirable to develop fiber amplifiers with wider bandwidths.
One way to expand the bandwidth of the optical fiber amplifiers is to use combine fiber amplifiers with different operating spectral ranges. For example, a fiber amplifier, e.g., a silica-based EDFA, in the L-C spectral band may use an EDFA in the C band from about 1530 nm to about 1560 nm and another EDFA in the L band from about 1570 nm to about 1600 nm a parallel EDFA configuration. In operation, input WDM channels may be split into C-band channels for amplification in the C-band EDFA and L-band channels for amplification in the L-band EDFA. The amplified C-band channels and L-band channels are then combined or xe2x80x9cstitchedxe2x80x9d together to produce amplified channels in the L-C band from about 1530 nm to about 1600 nm.
This application includes techniques and devices that combine two or more parallel fiber amplifiers of different operating spectral ranges with sampled Bragg gratings such as fiber Bragg gratings (FBGs) to achieve a wide operating spectral range for amplifying optical signals. A sampled fiber Bragg gratings may be designed to produce multiple Bragg reflection bands over a wide spectral range, e.g., over tens of nanometers, while each Bragg reflection band has a narrow bandwidth to carry a single WDM channel with a sharp roll-off at the band edge (e.g., a fraction of one nanometer). At last two sampled FBGs with about the same channel spacing may be used to split the input WDM channels for amplification by different fiber amplifiers and to combine amplified WDM channels from different fiber amplifiers together to produce the final amplified WDM channels.
This use of sampled FBGs may substantially reduce or eliminate the unusable spectral region between or in the is overlapping region of two fiber amplifiers with adjacent and different operating spectral ranges. In addition, the sampled FBGs effectively suppress amplified-spontaneous emission outside the WDM channel frequencies.
In one implementation, at least two sampled FBGs with about the same channel spacing but different operating spectral ranges may be used, where the Bragg reflection band at the highest channel frequency in the sampled FBG with a lower spectral range may be one channel spacing less than the Bragg reflection band at the lowest channel frequency in another sampled FBG with a higher spectral range. In another implementation, at last two sampled FBGs with about the same channel spacing and operating spectral ranges may be used.